A known seat apparatus for a vehicle includes a seat belt for fixedly retaining an occupant to a seat of the vehicle. Upper and lower belt anchors supporting both ends of the seat belt, respectively, are generally arranged at a body frame of the vehicle; however, the upper and lower belt anchors may be arranged at a rear portion of a seat frame in order to allow the seat to move or slide to various selectable positions. In case of a vehicle collision, the seat belt is pulled forward by a large tensile force caused by an inertia force acting on the occupant, thereby applying a large load to the upper and lower belt anchors. As a result, a moment caused by the load is generated at the seat frame. That is, an upward detaching load detaching the rear portion of the seat frame from a floor of the vehicle and a compressive load compressively deforming a front portion of the seat frame in a direction toward the floor of the vehicle are generated. Accordingly, it is extremely important for safety reasons to secure the mechanical strength of the seat apparatus against upward detaching and compressive loads of a movable mechanism supporting the seat frame.
For example, a seat apparatus for a vehicle disclosed in JP2004-122919A (hereinafter referred to as Patent Document 1) includes a longitudinal movement mechanism (longitudinal sliding mechanism) and a width-directional movement mechanism (width-directional sliding mechanism). The longitudinal movement mechanism includes a pair of lower members fixed at a floor of the vehicle and a pair of upper members engageable with the respective lower members and movable in a longitudinal direction of the vehicle. The width-directional movement mechanism includes front and rear connecting members (width-directional lower rails) and front and rear moving members (width-directional upper rails). The front and rear connecting members are arranged perpendicular to the upper members so as to cross-connect therebetween. The front and rear moving members are engageable with the respective front and rear connecting members and movable therealong in a width direction of the vehicle. The front and rear moving members of the width-directional movement mechanism are slidably connected to the front and rear connecting members of a seat, respectively. The front and rear connecting members of the width-directional movement mechanism are fixed to attachment members fixedly welded to side inner surfaces of respective front and rear portions of the pair of upper members of the longitudinal movement mechanism. Each of the attachment members has an L-shaped cross-section. In addition, an engaging hook of a first detachment prevention plate externally fixed to each of the front and rear connecting members is engaged with an engaged hook of a second detachment prevention plate externally fixed to each of the front and rear moving members, thereby increasing the strength of the width-directional movement mechanism against an upward detaching load.
However, in the seat apparatus according to Patent Document 1, the front and rear connecting members (width-directional lower rails) are fixed to the attachment members having the L-shaped cross-section and such attachment members are fixed to the side inner surfaces of the respective front and rear portions of the upper members of the longitudinal movement mechanism. The side inner surfaces of the upper members of the longitudinal movement mechanism are fixed to ends of a cantilever mechanism (one-end supporting mechanism). That is, the attachment members are supported to the fixed ends by means of the one-end supporting mechanism. Accordingly, when a large load acts on the front and rear connecting members, a large bending moment is applied to the side inner surfaces that are the fixed ends for the front and rear connecting members, thereby deforming the upper members. Further, strength against a detaching load is taken into consideration for the seat apparatus according to Patent Document 1. However, a point to which a load is applied (engagement point serving as a weight point between the engaging hook of the first detachment prevention plate and the engaged hook of the second detachment prevention plate) is deviated from a supporting center point for supporting the load in the width direction of the front and rear connecting members (width-directional lower rails). Accordingly, a large bending moment due to the above-mentioned load acts on the rear connecting member, thereby deforming the rear connecting member in an upward direction. Meanwhile, a large bending moment due to the above-mentioned load acts on the front connecting member, thereby compressively deforming the front connecting member in a downward direction. Furthermore, it is required to sufficiently increase a thickness of each of the first and second detachment prevention plates in order to increase the strength of the front and rear connecting members against such upward and downward loads. Such case leads to an increase of the weight of the seat apparatus.
A need thus exists for a seat apparatus for a vehicle, which is not susceptible to the drawback mentioned above.